How do I save the apps I purchased for my imac?

Well, I can be more specific since I have so far only purchased one app. Sorry, but I'm not used to downloading mac apps so this is a newbie question. I purchased a copy of Mental Case or my imac (and another copy for my ipod.) I haven't been using Time Machine or any other back up. Ready to remedy that. But I'm thinking of doing a clean install of Snow Leopard and doing a full backup right afterwards with Time Machine and/or SuperDuper. Will I need to purchase another copy of Mental Case? If not, what do I do to save the copy of Mental Case that I bought so that I can reinstall it once I have the clean install of Snow Leopard done? I don't know how this process works. Will it just force me to uninstall it when I burn it to a disc? Crap I am totally clueless and Mental Case is too bare bones to explain it.

Once I have an image of Snow Leopard I will also buy Lion and back that up, assuming Mental Case works with Lion.

Well, I can be more specific since I have so far only purchased one app. Sorry, but I'm not used to downloading mac apps so this is a newbie question. I purchased a copy of Mental Case or my imac (and another copy for my ipod.) I haven't been using Time Machine or any other back up. Ready to remedy that. But I'm thinking of doing a clean install of Snow Leopard and doing a full backup right afterwards with Time Machine and/or SuperDuper. Will I need to purchase another copy of Mental Case? If not, what do I do to save the copy of Mental Case that I bought so that I can reinstall it once I have the clean install of Snow Leopard done? I don't know how this process works. Will it just force me to uninstall it when I burn it to a disc? Crap I am totally clueless and Mental Case is too bare bones to explain it.

Once I have an image of Snow Leopard I will also buy Lion and back that up, assuming Mental Case works with Lion.

3.06 Ghz Intel Core 2 Duo
4 GB 1067 Mhz DDR3

If you bought them from the Mac App Store, you can re-download them from the MAS after you wipe the drive.